A telecommunications network typically consists of several network elements and the trunk lines that connect these elements to one another. The network can be synchronous, such as a Synchronous Digital Hierarchy (SDH) network, or plesiochronous such as a Plesiochronous Digital Hierarchy (PDH) network.
The PDH networks include, among others, the digital mobile communications network which is used as an example to illustrate the application network of the invention. References are made to FIG. 1 which shows a simplifled diagram of the GSM network (Groupe Speciale Mobile) from the point of view of transmission. The network subsystem NSS consists of the mobile switching center MSC through whose system interface the mobile communications network connects to other networks, such as the public switched telephone network PSTN.
The network subsystem NSS is connected via the interface A to the base station subsystem BSS which consists of base station controllers BSC, each of which controls the base transceiver stations BTS connected to these controllers. The interface between the base station controller and the base stations connected to it is an A bis interface. The layer 1 physical interface between the mobile switching center, base station controller BSC, and the transmission parts of the base station is a 2 Mbit/s line, i.e. 32 time slots of 64 kbit/s (=2048 kbit/s). The RF parts of the base stations, on the other hand, are completely controlled by the base station controller BSC, mostly consisting of transceivers TRX which implement the radio interface to the mobile station.
The network formed by the base stations controlled by the base station controllers can be star-shaped as the system on the left in the Figure, in which case each base station is directly connected to the base station controller, or a loop which consists of base stations chained to one another and which terminates either in one of the base stations as in the system shown in the middle of the Figure, or in the base station controller. The network can also be a chain, in which case a trunk line leads from the base station controller to one of the base stations, from that to another base station, etc., as shown in the base station system on the right in FIG. 1. In a mobile communications network any of the above base station network types or a combination of them can be used as necessary.
As in other PDH networks, in a mobile communications network also the network elements should operate in synchronization with one another to avoid frame slips. Additionally, the base stations require an exact synchronization to form the correct kind of signal for the air interface and to keep it with sufficient precision in the frequency band reserved for it.
There are several different methods available for synchronizing the network elements of a PHD telecommunications network with one another. A separate clock signal can be input into the network element from an accurate clock source which can be shared by the entire network. Each network element can have a separate accurate clock source, or they can receive synchronization, for example, through the GPS satellite system. However, because all of the above methods are relatively expensive, the most common way is to use the so-called master-slave synchronization. In master-slave synchronization the higher element of the network hierarchy provides the synchronization to a lower element as part of the transmitted signal, in which case an accurate timing source is only needed in the topmost network element of the hierarchy. In the case of the mobile communications network, the master clock is in the mobile switching center MSC which provides the clock signal to base station controllers which further transmit the clock signal to the base stations controlled by them. The network elements receive the clock frequency and phase directly from the bit speed of the trunk line at 2 Mbit/s. In the case of a chained base station network, the clock signal thereby travels via the route MSC.fwdarw.BSC.fwdarw.BTS1.fwdarw.BTS2.fwdarw.etc.
It should be noted that a network element must not receive a synchronization signal simultaneously from several directions. Even if the base station network were a loop, as far as the master clock signal is concerned, it is not a loop, but the loop has been interrupted at a point so that the base stations only receive their clock signal from one direction. However, as far as voice and data transfer are concerned, the loop is not interrupted, in which case a fault in one of the links does not affect the speech transfer, although the synchronization with the master clock is lost.
The drawback in the master-slave synchronization in a mobile communications network is that in network fault situations, for example, when the fault is in the connection MSC.rarw..fwdarw.BSC, the isolated part of the network, for example, from the BSC down, synchronizes itself according to the clock formed by some network element and because of this the base stations drift outside their frequency band. However, the base station can maintain its synchronization and frequencies even through long interruptions in the synchronization chain. However, a harmful situation is created when a base station corrects its own frequency by thinking that the synchronization comes from a high-quality clock source (of nominal frequency), while this is not actually the case.
The drifting of a base station frequency outside of the frequency band may cause, among other things, the operator's network and that of a competing operator to go to an unusable state for several hours. The unusable state causes economic losses for the operator through lost charges, possible compensations paid to the other network operator, and new subscribers lost because of the loss of repute.
In the known master-slave method the base station deduces that synchronization is allowed if the base station has a communications connection with the base station controller. The synchronization occurs repeatedly in 20 minute intervals so that the phase of the base station clock is compared n times with the phase of the clock received from the network (MSC.fwdarw.BSC.fwdarw.BTS), and the average is calculated for these phase differences. If the phase difference according to the final average were to require too large a correction, the result is discarded. The requirement for the activation of the synchronization event and for its continuation is thereby that the communications connection is held throughout the entire synchronization phase.
The weakness of this known method is that in certain network fault situations or in a combination situation of several simultaneous errors the signals are connected through and the communications connection in the interval BSC.rarw..fwdarw.BTS operates but some of the base stations have lost their synchronization. The situation described above may be generated, for example, in a loop network when two one-way faults are simultaneously in effect and if the signal which contains the alert for the far end is not allowed as a clock source. Similarly during network modification operations and especially after them one of the intervening nodes might be left on its internal clock without anybody noticing it.
The reliability of master-slave synchronization can be improved by adding a special control bit MCB (Master Clock Bit) in the clock signal. This issue is described by referring to FIG. 2. The control bit MCB is added to the clock signal at the base station BTS0 of the loop network which is connected to the base station controller BSC and from which the loop thereby starts and in which the loop terminates. This control bit is in the agreed logical state, for example, 0. When one of the slave stations in the loop network, for example, base station BTS1 receives the MCB control bit whose value is 0, the base station knows that the clock signal comes from the true master clock. The transmission unit TRU of the base station synchronizes itself with the master clock and transmits the clock signal as a reference signal further via the internal bus B of the base station to the clock oscillators implemented, for example, by using the PLL connection located in the other functional parts of the base station. Additionally, the base station transmits the clock signal with its MCB bit set to "0" to the next base station BTS2. The transmission unit TRU of the next base station detects from the MCB bit that the received clock signal is the true signal, and it transmits the clock signal and the MCB bit further to the next base station BTS3, etc. The base station BTS2 cannot synchronize to the clock signal that arrives from the direction 2 (from the direction of BASE STATION3) because this signal has been looped in BASE STATION3, and this is indicated to BTS2 by using the bit LCB=1.
In each base station input directions have been assigned to the clock inputs of the transmission unit, two of which are indicated in the figure by numbers 1 and 2. If there are more directions, they are numbered consecutively. The directions are prioritized so that direction 1 is the direction, from which the clock signal comes, to which the system primarily synchronizes, and direction 2 is the direction to which the clock signal is transmitted and to which the system synchronizes secondarily. One of the clock inputs in the priority list is the internal clock of the transmission unit. The units are connected via their clock inputs in the loop in the manner shown in FIG. 2.
If the link between two base stations of the loop, for example between base stations BTS1 and BTS2, is interrupted, the first base station BTS2 located after the interruption does not receive the clock signal. In this case the transmission unit TRU of the base station starts using its internal clock and transmits it further as the clock signal, but changes the MCB bit to "1". BTS2 cannot synchronize to the clock signal that arrives from direction 2 (from the direction of BTS3) because BTS2 is looped via BTS3 and this is indicated in the feedback by using the bit LCB=1. From the value of the MCB bit the next base station BTS3 detects that the clock signal being used is not the original clock signal, although the base station synchronizes to this clock signal. As a summary it can be noted that the MCB bit is transmitted from the main station of the network, in a mobile communications network from one of the base stations in the chain formed by the base stations in 0 state, and as the bit progresses through the network, it indicates whether the clock included in the signal in question originates in the main station of the network (MCB=0) or if a fault situation has forced one of the network elements to start using its internal clock (MCB=1), in which case the network element located after the fault is synchronized to this clock.
The addition of the MCB bit to the clock signal in the mobile communications network is sufficient, if the base station network is chained. In the case of a fault the network automatically divides at one of the links into two parts, and all network elements located after the fault lock to the internal clock of the base station located nearest to the fault.
In a looped network, in the case of one fault, it is possible to bring the master clock signal to the base stations from the other direction, in other words, from direction 2; see FIG. 2. In this case the base stations must be told in one way or another that the clock signal that arrives from this direction is, after all, the master clock and the base stations must synchronize to it. This can be implemented by adding another special monitoring bit LCB (Loop Clock Bit) in addition to the MCB bit to the clock signal that leaves from the base station BTS0 which operates as the main station; see FIG. 2. Both monitoring bits are, as the signal leaves the main station, in 0 state. The value 0 of the LCB bit indicates to the receiving base station BTS1, . . . , BTS3 that the clock signal is not a returned looped clock signal. In the normal state each base station of the loop transmits the clock signal further and keeps both the MCB bit and the LCB bit in 0 state. Additionally, the base station also transmits the clock signal back in the direction from which it arrived, and keeps the MCB bit in state 0 as a signal that the base station is locked to the original master clock signal. The LCB bit, on the other hand, is changed to 1 which indicates that the clock signal is a returned master clock signal in which case the preceding base station does not lock to this signal. If the value of the LCB bit is 0 and the value of the MCB bit is 0, it means that the base station must synchronize itself to this clock signal that arrives from direction 2, as it arrives from the master clock.
The implementation according to the prior art described above does not solve the problem of how to maintain synchronization when the base station network is a looped network and two simultaneous faults occur in the loop. In FIG. 3, the first fault between BTS0 and BTS1 prevents the clock signal from getting from direction 1 to the transmission unit of the base station BTS1 and the second fault between the BTS3 and BTS0 causes the transmitting of the far end alarm bit FEA from BTS0 in the direction of BTS3, because of which BTS3 is not allowed to lock to the signal arriving from the direction in question. The difficulty is caused by the fact that as far as voice/data traffic is concerned, the loop is a true loop and in the case of an interruption, the connection to the base station controller is automatically formed via the other branch. For synchronization the loop is not a true loop, but it is formed by the chain BTS0, . . . , BTS3. During an interruption the traffic can, therefore, continue without interruption but the base stations BTS1 to BTS3 do not receive the master clock signal.
At first the base station BTS1 reverts to using the internal clock and transmits both the MCB bit and the LCB bit in state 1 to direction 2, in other words, to the base station BTS2. BTS2 also reverts to using the internal clock and transmits the aforementioned bits without changes further in direction 2 to base station BTS3. BTS3 reverts to using the internal clock and transmits the clock signal back including the MCB bit in state 1 (=the signal is not the master clock signal) and the LCB bit in state 0 (=the signal is not the returned clock). This signal arrives to the base station BTS2 from the direction which is second in the priority list so the base station locks itself to this clock signal and transmits the signal further back to the base station BTS1. The values of the monitoring bits are 1/0 so BTS1 also locks to this clock signal. The final result conforms to FIG. 3 where the base stations BTS1 to BTS3 are synchronized to the internal clock signal of the base station BTS3. The issue described above can also be seen so that as a result of the first fault the network is already synchronized as a chain. When the second fault occurs, the device of the chain that is closest to the fault, BTS3, reverts to using the internal clock and transmits MCB as 1 to the other devices of the chain which keep their synchronization in the direction in question.
The network administrator receives no notification of the fact that a part of the network is no longer in synchronization. The situation may continue for weeks, or even for months, until the part of the network in question has drifted badly away from its frequency band.
The objective of this invention is a method by which the synchronization of a chained and looped base station network reverts quickly to use of the master clock in different fault situations and which ensures that the base station receives reliable information about the quality of the synchronization signal in use. In this manner it is ensured that the fault situation of a master-slave synchronized base station network described above does not cause the base station to drift away from its frequency band.